It has heretofore been known that the thermal stability and barrier properties of oriented blow molded containers of polyethylene terephthalate are significantly increased by heat setting. Typical processes for heat setting are shown in U.S. Pat. Nos. 4,476,170, 4,512,948 and 4,522,779.
In U.S. Pat. Nos. 4,476,170 and 4,512,948, there is disclosed an article and a process of making an oriented and heat set blow molded container of polyethylene terephthalate. In the process, a perform preheated to a temperature suitable for orientation is biaxially stretched in a blow mold and then while the hollow container is still in contact with the blow mold walls, the article is raised to a higher heat setting temperature preferably in the range of 200.degree.-250.degree. C. (except for the neck) thus heat setting the container, and while the container is still at a shrinkage resisting pressure exceeding atmospheric, it is cooled in the same mold to a temperature at which it maintains its shape when not pressurized but not below 100.degree. C. It is also particularly disclosed that this cooling step can be done in the air outside the mold while maintaining internal pressure. According to these patents, when the heat setting temperature of the hot mold ranges from 220.degree.-250.degree. C. and the quenching temperature is not below 100.degree. C., higher onset-of-shrinkage temperatures are obtained.
In U.S. Pat. No. 4,522,779, there are disclosed improved plastic containers and a process for their production. In the first embodiment, a container is blow molded in a first hot blow mold, then reblown to a larger size in a second cold mold of larger volume than the first hot mold. Such containers are stated as having improved mechanical properties, particularly very high hoop yield stresses. However, the utilization of a larger volume cold mold substantially reduces the thermal stability. In the second embodiment, a container is blow molded in a hot blow mold, then reblown to a larger size in a second hot blow mold where it is blown to the confines of the second mold and the container is then removed from the second hot mold and transferred to a third cold mold and cooled to room temperature while maintaining internal pressure. In a further embodiment, the container is blow molded in a first hot mold, reblown in a second hot mold, and thereafter the second mold is cooled to cool the container.
U.S. Patent No. 4,385,089 (British Patent Specification 1,604,203) is directed to heat set biaxially oriented hollow articles and states that the preform or parison should be heated at least to biaxially orientation temperature and maintained in closed contact with a hot mold which is at a temperature of up to 40.degree. C. above the minimum orientation temperature. In one embodiment, the resultant molded hollow article is moderately cooled causing a temperature drop of 10.degree.-30.degree. C. by introducing cooling vapor or mist into the hollow article, interrupting the cooling vapor, and opening the mold. In another embodiment, the heat set article is allowed to shrink freely and then reblown in the same hot mold or in a separate cooled mold. The patent calls for a heat setting temperature of 40.degree. C. above the orientation temperature which limits thermal stability and barrier properties.
According to this patent, the temperature of the hot mold should be maintained between 30.degree. and 50.degree. C. above the minimum orientation temperature of the plastic material. Otherwise, it is stated there are numerous disadvantages including lowering of the production rate, the danger of the appearance of major distortion and major shrinkage on mold release, the disadvantage inherent in heating metal molds to very high temperatures and keeping them at such temperatures, and the danger of crystallization which would cause a loss of transparency. Further, in accordance with this prior patent, excessive shrinkage is to be avoided and generally the temperature drop of 10.degree. to 30.degree. C. should be made. Accordingly, such a method precludes obtaining a degree of heat setting which would produce thermal stability at higher temperatures as may be required in filling the container with various products. In addition, such a method will preclude obtaining the higher degrees of crystallinity and resultant high barrier properties which are required for some products.
In copending application Ser. No. 923,503, filed Oct. 27, 1986, there are disclosed improved plastic containers and a method for their manufacture. In this method, the container is blow molded in the first hot mold which is maintained at 130.degree.-250.degree. C., the container is in contact with the mold surface for a short period of time (1-10 sec.) which is sufficient to induce partial crystallization. The container is then maintained at lower internal pressure to prevent significant shrinkage and then the container is rapidly transferred into a cold mold having substantially the same volume or smaller volume than the hot mold. The temperature of the cold mold is maintained at 1.degree.-100.degree. C. The container is quenched in the cold mold. The method results in a thermally stable container which has higher onset-of-shrinkage temperature and higher mechanical properties as required for hot fill applications. The method also provides lower cycle time.
Where the blown container is moved rapidly from a hot blow mold to a cold mold, the container will become deformed by tilting relative to the finish or neck by which it is held. In addition, where the base of the container is complex such as having axially projecting portions so that it is free standing or has an inverted bottom, the transfer, while the interior of the container is under pressure, tends to deform the bottom and cause it to revert to a hemispherical bottom. This tends to occur both when the blown container is maintained stationary and when the container is moved from a hot mold to a cold mold.
Accordingly, among the objectives of the present invention are to provide a method for heat setting containers which will obviate deformation of the bottom and permit the container to be transferred more rapidly between the hot mold and the cold mold, thereby reducing the cycle time.
In accordance with the invention, the method for making a partially crystalline, biaxially oriented heat set hollow plastic container from a hollow parison having an open end and a closed end comprising engaging the open end of a plastic parison which is at a temperature within its molecular orientation temperature range, positioning a mold base in axial alignment with said engaged hot parison, enclosing a hot mold about the mold base, the mold being at heat setting temperature, expanding the plastic parison within the hot mold and mold base by internal pressurization to induce biaxial orientation of the plastic parison and force the plastic parison into intimate contact and conformance with the hot mold and to maintain contact by such internal pressurization between the mold and the biaxially oriented container for a time sufficient to induce partial crystallization in the biaxially oriented container, lowering the internal pressure in the container, opening the hot mold while maintaining engagement of the open end and engagement of the mold base with the blown hollow container, maintaining a lower internal pressurization of the container to prevent significant shrinkage, and cooling the container while maintaining engagement of the open end and engagement of the mold base with the container and maintaining pressure in the container at least sufficient to prevent shrinkage.
In a preferred form, the parison is heated to the orientation temperature and maintained for a given period of time at that temperature in order to equilibrate the inside and outside temperature of the parison, the parison is placed on a neck clamping and blow pin assembly and transferred to a hot blow molding station and a mold base is shifted into position and the hot mold is closed about the clamping and blow pin assembly and mold base, the parison is blown through the neck and heat set for a given period of time, the blowing pressure is then reduced to a lower transfer pressure, the hot mold is opened, and the mold base and clamping and blow pin assembly through which it has been blown are moved together to transfer the blown article to a cold mold, the cold mold is closed about the mold base, neck clamping and blow pin assembly and container and the container is pressurized to a higher pressure than the transfer pressure, quenched, and then the pressure is exhausted to atmosphere pressure, after which the cold mold is opened, the article removed, and the mold base is returned with the clamping mechanism to the initial position. Alternatively, the mold base and clamping assembly are maintained stationary and the hot and cold molds are moved into coacting relationship with the mold base and clamping assembly.